Aspheric Hip Bearing Couple

ABSTRACT

A joint prosthesis device comprises a head configured to fit within a cup and in one embodiment includes a first member for attachment to a first bone in a joint and including a first articulation surface portion defined by a first radius of curvature and a second member for attachment to a second bone in the joint and including a second articulation surface portion defined by a second radius of curvature and a third articulation surface defined by a third radius of curvature, wherein each of the second radius of curvature and the third radius of curvature has a length that is different from the length of the first radius of curvature by less than 0.05 millimeters and wherein the origin of the second radius of curvature is not coincident with the origin of the third radius of curvature.

FIELD

This application relates to the field of prosthetic devices, andparticularly joint prostheses comprising head and cup arrangements.

BACKGROUND

A common orthopedic joint prosthesis includes a ball and cuparrangement. For example, hip joints typically comprise a roundedfemoral head and an acetabular cup. The rounded femoral head is providedon a stem that is configured to engage the intramedullary cavity of thefemor and secure the head on the femor. The rounded femoral headincludes a convex surface configured to engage a concave surface on theacetabular cup. The acetabular cup is configured for implantation on theacetabulum of the pelvis. When the rounded femoral head is receivedwithin the acetabular cup, a ball and socket joint is provided.

In order to reduce wear between the components of the joint prosthesis,the components are manufactured such that the clearance between thebearing surfaces is minimized. The term “clearance” is often used inreference to a “diametral clearance” of the joint prosthesis. Thediametral clearance between bearing surfaces is generally considered tobe the difference in the diameter defining the bearing surface of theball and the diameter defining the bearing surface of the cup.

While minimal diametral clearance between the bearing surfaces isdesired, at least two factors limit the reduction of clearances. First,manufacturing tolerances generally limit the extent to which clearancesmay be reduced. For example, for diametral clearances below the 15-30micron range, it has been observed that imperfect formation of thefemoral head and the acetabular cup contributes to local interferencesand small deformations that result in wear.

Second, acetabular cup deformation during implantation into theacetabulum also limits the degree to which clearances may be reduced ina hip joint prosthesis. This deformation generally occurs near theequatorial lip of the acetabular cup. For substantially spherical cupand head arrangements, reduction in clearances near the pole of the headalso means reduction in clearances near the equatorial lip. In otherwords, when the head and the cup of a hip prosthesis are substantiallyspherical, the small clearances near the pole of the head are also foundin the region near the equatorial lip of the cup. Thus, when cupdeformation occurs near the equatorial lip in a low clearance sphericaldesign, interference is likely to occur between the equatorial lip ofthe cup and the ball.

One way to reduce clearance complications resulting from acetabular cupdeformation is to provide a conformal region having a small clearancenear the center of the primary articulation area of the femoral head,and a peripheral region surrounding the conformal region, wherein theperipheral region has a significantly greater clearance than theconformal region, including a significantly greater clearance near thelip of the cup. With this arrangement, deformations near the equatoriallip of the acetabular cup are less likely to result in obstruction withthe femoral head because of the increased clearance near the equatoriallip. Although several of these arrangements have been provided in thepast, they have not provided optimal solutions. In particular, many ofthese arrangements include peripheral regions surrounding the conformalregion where the clearances in these peripheral regions quickly divergefrom the relatively small clearances in the conformal zone. However,when the clearance in the peripheral region is too great, significantwear may result.

Accordingly, what is needed is a joint prosthesis configured to avoidinterference between the ball and cup even if the equatorial region ofthe cup is deformed during implantation. It would also be advantageousif the clearance between the ball and cup could remain relatively loweven in a peripheral region surrounding the conformal region.

SUMMARY

A joint prosthesis device comprises a head configured to fit within acup and in one embodiment includes a first member for attachment to afirst bone in a joint and including a first articulation surface portiondefined by a first radius of curvature and a second member forattachment to a second bone in the joint and including a secondarticulation surface portion defined by a second radius of curvature anda third articulation surface defined by a third radius of curvature,wherein each of the second radius of curvature and the third radius ofcurvature has a length that is different from the length of the firstradius of curvature by less than 0.05 millimeters and wherein the originof the second radius of curvature is not coincident with the origin ofthe third radius of curvature.

In another embodiment, a prosthetic device includes a cup including aconcave surface defining a cavity, the concave surface defined by atleast one radius of curvature (R_(C)), and a head including an outersurface and configured to fit at least partially within the cavity, theouter surface including a cap portion defined by a cap radius ofcurvature (R_(P)) and a toroidal portion located about the cap portionand defined by a toroidal radius of curvature (R_(T)), wherein the capportion is configured to conform with the concave cup surface and theR_(C) is less than 0.05 millimeters longer than the R_(T).

In a further embodiment, a prosthetic device includes a cup including aconcave surface defining a cavity, the concave surface defined by atleast one radius of curvature (R_(C)), and a head including anarticulation portion configured to fit at least partially within thecavity, the articulation portion including a toroidal portion defined bya toroidal radius of curvature (R_(T)) having a circular origin and acap portion defined by a cap radius of curvature (R_(P)), wherein theR_(C) is less than 0.05 millimeters longer than the R_(T).

In yet another embodiment, a prosthetic ball for use in a ball and cupjoint system includes a spherical cap articulation portion defined by aradius of curvature, and a toroidal articulation portion defined by aportion of the inner surface of a spindle torus.

The above described features and advantages, as well as others, willbecome more readily apparent to those of ordinary skill in the art byreference to the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of various components of a hipprosthesis including an acetabular cup, a femoral head, and a femoralstem in accordance with principles of the invention;

FIG. 2 shows a cutaway view of the hip prosthesis of FIG. 1 assembledand implanted in a pelvis and femur;

FIG. 3 shows a cross-sectional view of the femoral head and acetabularcup of FIG. 1;

FIG. 4 shows a diagrammatic representation of the radii which define theinner surface of the cup, a primary contact zone and a toroidal zone inaccordance with principles of the invention;

FIG. 5 depicts a perspective view of a slice of the femoral head of FIG.3 showing the relative positions of the origin for the radius ofcurvature of the cap portion of the femoral head and the circular originof the radius of curvature of the toroidal section;

FIG. 6 shows a cross sectional view of a spindle torus;

FIG. 7 shows a three-dimensional representation of the bottom half ofthe torus of FIG. 6 including the apple shaped or outer surface and thelemon shaped or inner surface of the torus;

FIG. 8 shows a diagrammatic representation of the definition of theouter surface of a femoral ball using the lemon shape or inner surfaceof a spindle torus and an arc of a circle centered below the axisdefined by the center of the circle (the circular origin) which definesthe spindle torus;

FIG. 9 shows a diagrammatic representation of the circles of FIG. 8 withthe center of the circle used to define the arc or cap portion closer tothe axis defined by the center of the circle (the circular origin)thereby decreasing the size of the cap portion as compared with FIG. 8;

FIG. 10 shows a diagrammatic representation of the circles of FIG. 8with the center of the circle used to define the arc or cap portion atthe same distance away from the axis defined by the center of the circle(the circular origin) as shown in FIG. 8 but with the diameter of thecircular origin reduced (i.e. the apple shaped portion is more circular)thereby increasing the size of the cap portion as compared with FIG. 8;

FIG. 11 shows a cross-sectional view of the femoral head and acetabularcup of FIG. 1 with the femoral head rotated within the acetabular cupand contacting the acetabular cup at the opening of the acetabular cup;

FIG. 12 shows a cross-sectional view of an alternative embodiment of anacetabular cup with a diagrammatic overlay showing the use of a toroidalsurface within the acetabular cup in accordance with principles of theinvention; and

FIG. 13 shows a cross-sectional view of an alternative embodiment of anacetabular cup with a diagrammatic overlay showing the use of a toroidalsurface within the acetabular cup wherein the cap portion and toroidalportion of the acetabular cup are not centered within the acetabular cupin accordance with principles of the invention.

DESCRIPTION

With reference to FIG. 1, a prosthetic device in the form of aprosthetic hip joint 100 is shown in a disassembled configuration. Theprosthetic hip joint 100 includes an acetabular cup 102 and a femoralcomponent 104. The femoral component 104 includes a femoral head 106 (or“ball”), and a femoral stem 108. The femoral head 106 is configured forattachment to the femoral stem 108. The femoral head 106 is alsoconfigured to slideably engage the acetabular cup 102.

The acetabular cup 102 is the part of the prosthetic hip joint 100 thatforms the socket of a ball-and-socket structure. The acetabular cup 102includes a convex outer surface 110 configured for engagement with apatient's acetabulum and a concave interior surface 112 configured toengage the femoral head 106. The cup 102 includes a lip 114 whichdefines a rim in a peripheral region and which extends between theconvex outer surface 110 and the concave interior surface 112.

The convex outer surface 110 of the acetabular cup 102 may be providedas part of a shell including a biocompatible material. In at least oneembodiment, the shell is comprised of a relatively rigid material, suchas a biocompatible metal or ceramic. For example, the shell may becomprised of titanium or cobalt chrome. The concave interior surface 112of the cup 102 may be in the form of a liner that provides a bearingsurface for the acetabular cup 102. The liner may be comprised of abiocompatible material that offers a low coefficient of friction, suchas polyethylene. Alternatively, the liner may be comprised of a metal orceramic. While exemplary materials for the acetabular cup 102 have beenoffered herein, one of skill in the art will recognize that numerousother biocompatible materials may be used as are known in the art.

The femoral component 104 is used to replace the natural head of afemur. To this end, the femoral head 106 includes a generallyball-shaped outer surface 116 designed and dimensioned to be received atleast partially within the cavity defined by the concave interiorsurface 112 of the acetabular cup 102. The femoral head 106 includes agenerally conical bore 118 which is used to fix femoral head 106 to aMorse taper 120 on the neck 122 which extends from the femoral stem 108.The femoral component 104 is comprised of a relatively rigidbiocompatible material such as a ceramic or metal. For example, the ball106 may be comprised of cobalt chrome or stainless steel. Whileexemplary materials for the femoral component 104 have been offeredherein, one of skill in the art will recognize that numerous otherbiocompatible materials may be used as are known in the art.

As shown in FIG. 2, the prosthetic hip joint 100 may be implanted in apatient by securing the acetabular cup 102 in the acetabulum 124 of thepelvis 126. Also, the femoral component 104 is secured to the femur 128by inserting the femoral stem 108 within the intramedullary cavity 130of the femur 128. The femoral head 106 which extends from the neck 122is brought into slideable contact with the acetabular cup 102 such thatthe femoral head 106 is allowed to articulate within the acetabular cup102. This slideable relationship provides for a ball and socket typejoint.

An enlarged cutaway view of the acetabular cup 102 showing the femoralhead 106, with the head 106 slightly removed from engagement with thecup 102 is shown in FIG. 3. The configuration of the head 106 definesdifferent zones or regions for the prosthesis, including a primarycontact zone A and a toroidal zone T.

The term “primary contact zone” refers to a region of the head 106 whichprovides the main contact area between the head 106 and the cup 102 formost joint movements once implanted in a patient. Accordingly, withreference to FIG. 3, the convex bearing surface 116 of the head 106primarily articulates with the concave bearing surface 112 of the cup102 within the primary contact zone A. Some contact, however, occursbetween the head 106 and the cup 102 within the toroidal zone T,particularly with certain extra-ordinary movements by the patient.

The primary contact zone A is shown as lying within the region subtendedby the angle α having a vertex at an origin 140 of the spherical capportion. This means that the primary contact zone A is provided within aperimeter defined by the intersection of a cone 142 with the convexouter surface 116 of the head 106, the cone 142 having an apex 144 atthe origin 140 and an aperture (or “opening angle”) of α. As shown inFIG. 3, the cone 142 is symmetric about an axis 146 extending throughthe origin 140. The toroidal zone T extends from the primary contactzone A to the conical bore 118.

Studies such as Bergmann, et al., “Hip contact forces and gait patternsfrom routine activities,” J. Biomech., 2001, 34(7), 859-871, have shownthat contact predominantly occurs in an area defined by opening anglesbetween 85 and 145 degrees. Accordingly, while the α in this embodimentis 95 degrees other opening angles between 85 and 145 degrees may beused. Selection of opening angles between 95 and 125 degrees provide forgood radial clearance which is discussed below.

The acetabular cup 102 is shown in FIG. 3 centered upon and symmetricwith respect to an apex 148, which is the deepest portion of the cup102, in the coronal plane. In particular, the apex 148 of the concavebearing surface 112 of the cup 102 is shown in FIG. 3 aligned with theaxis 146. When the cup 102 is in this position relative to the head 106,it is considered to be in a centered position. In practice, the cup 102and head 106 are generally aligned in the implanted position such thatthe apex 148 of the cup 102 is about thirty degrees off the axis 146 ofthe head 106 in the coronal plane and about fifteen degrees off the axis146 of the head 106 in the sagittal plane. For a spherical cup geometry,the articulation area on the head is independent of the cup orientation.

With continued reference to FIG. 3, the outer surface 116 of the head106 at any given point is defined by a radius of curvature (R_(H)). Thehead 106 does not form a perfect sphere, however, and the radius ofcurvature R_(H) is different at different points on the surface 116 ofthe head 106 as shown in FIG. 4. The radius of curvature in the primarycontact zone (R_(P)) in the embodiment of FIG. 3 is 18.035 mm, while theradius of curvature in the toroidal zone (R_(T)) is 18.0120 mm.

Moreover, as shown in FIG. 4, the origin of the Rp is located at theorigin 140. The origin of the R_(T), however, is defined by a circle 150shown in FIG. 5. FIG. 5 depicts a slice of the femoral head 106 takenalong the plane defined by the axis 146 and an axis 152 which isperpendicular to the axis 146 and which intersects the origin 140. Theportion of the circle 150 which is behind the slice of the ball 106 asdepicted in FIG. 5 is shown as a dashed line. The circle 150 has aradius of 0.0155 mm and lies within a plane that is located 0.0510 mmabove the origin 140 and positioned perpendicular to the axis 152.

Any given point on the outer surface 116 in the toroidal zone T isdefined by an R_(T) having an origin located on the point of the circle150 farthest away from the point being defined. For example, the arc 154of the surface 116 shown in FIG. 5 is defined by sweeping R_(T) from theposition shown as R_(T1) to the position of R_(T1)′ while maintainingthe origin of the R_(T) at the point 156. Similarly, the arc 158 of thesurface 116 of FIG. 5 is defined by sweeping R_(T) from the positionshown as R_(T2) to the position of R_(T2)′ while maintaining the originof the R_(T) at the point 160. Thus, the origin of the R_(T) shown inFIG. 4 is located at a point 0.0510 mm above the axis 152 and 0.0155 tothe left of the axis 146.

From a mathematical construct, the toroidal zone T is thus formed as thelemon of a spindle torus. A spindle torus is formed by the revolution ofa circle about an axis coplanar with the circle. A cross sectional viewof a torus 162 is shown in FIG. 6 while FIG. 7 is a three-dimensionalrepresentation of the bottom half of the torus 162. The torus 162 incross-section presents two overlapping circles 164 and 166. The centers168 and 170 of the circles 164 and 166 are points on a circle 172. Thecircle 172 is thus the circular origin of the torus 162 having a radiusof curvature 174 which is the radius of the circles 164 and 166. Theouter surface 178 of the torus is referred to as the “apple” shape whilethe inner surface 180 is referred to as the “lemon” shape.

As shown in FIG. 8, the lemon 180 and a circle 182 having a center 184located below the axis 186 defined by the centers 168 and 170 of thecircles 164 and 166, respectively can be used to define a cap 192. FIG.9 is identical to FIG. 8 with the exception that the origin 184 of thecircle 182 has been positioned closer to the axis 186 defined by thecircular origin of the outer surface 178. As is apparent from comparingFIG. 9 with FIG. 8, as the origin 184 of the circle 182 approaches theaxis 186, the cap portion 192 becomes smaller. The shape can be furthermodified by moving the origin 184 closer to one or the other of thecenters 168 and 170. Consequently, the location and extent of thediscontinuity between the cap portion 192 and the outer surface 180 canbe modified.

Thus, by moving the origin or center 184 closer to the axis defined bythe circular origin, the spherical cap portion 192 becomes smaller. Forexample, given a circular origin diameter of 0.031 millimeters, an R_(P)of 18.035 millimeters and an R_(T) of 18.0120 millimeters, a cap portionwith a 95 degree opening angle is obtained by positioning the origin ofthe spherical cap portion 0.051 millimeters below the plane of thecircular origin. In the event a cap portion with a 125 degree openingangle is desired using the same radii, one need only position the originof the spherical cap portion at about 0.08 millimeters below the planeof the circular origin.

FIG. 10 is identical to FIG. 8 with the exception that the diameter ofthe circular origin 172 of the outer surface 178 is reduced. Thus, twocenters 168 and 170 of the circles 164 and 166 are positioned moreclosely together. As is apparent from comparing FIG. 10 with FIG. 8, asthe two centers 168 and 170 of the circles 164 and 166 converge, thatis, as the diameter of the circular origin 172 is shortened, the shapeof the inner surface 180 becomes more circular, thereby increasing thesize of the cap portion 192. Consequently, the location and extent ofthe discontinuity between the cap portion 192 and the outer surface 180can be modified.

Moreover, while the circles 164, 166 and 182 are shown with identicalradii, the radius of the circle 182 may be shorter or longer than theradii of the circles 164 and 166 in certain embodiments. Similarly, theradius of the circle 182 may be the same, shorter or longer than theradius or radii of a particular cup.

Returning to FIG. 3, the acetabular cup 102 is defined by a radius ofcurvature (R_(C)). The R_(C) extends from the virtual center of the cup102, which as depicted in FIG. 3 is coincident with the origin 140, tothe concave inner surface 112 of the acetabular cup 102. The R_(C) isconstant for all points on the concave inner surface 112 such that theconcave inner surface 112 of the cup forms a hemisphere. The R_(C) inthis embodiment is 18.050 mm.

The radial clearance (R_(CL)) or difference between R_(C) and R_(H) at agiven point on the head 106 and the opposing point on the cup 102 (i.e.,on a given ray extending from the origin 140 of the head 106 to theconcave surface 112 of the cup 102) does not necessarily translatedirectly into a spatial clearance between the head 106 and the cup 102.For example, when the prosthesis 100 is implanted and the head 106 is ina centered position, the head 106 is in contact with the cup 102, eventhough the R_(CL) is 0.015 mm (R_(C) (18.050 mm)−R_(P) (18.035 mm)). Thevalue of R_(CL), however, is useful in quantifying the conformitybetween the surface of the ball 106 and the cup 102 which are incontact. For example, a small R_(CL) for a given contact area, i.e. lessthan 0.05 mm, generally provides lower wear rates. Accordingly, theprosthetic hip joint 100 maintains an R_(CL) less than 0.050 mmthroughout the primary contact zone A.

Additionally, the toroidal zone T provides increased clearance betweenthe ball 106 and the cup 102 at the lip 114. With reference to theembodiment of FIG. 3, the acetabular cup 102 is exactly hemispherical.Thus, the width of the cup 102 at the plane defined by the lip 114 isthe widest portion of the cup 102. Accordingly, when the ball 106 iscentered within the cup 102 and in contact with the cup 102 along theaxis 146, the origin 140 is located 0.015 mm above the plane defined bythe lip 114. Thus, the widest diameter defined by the toroidal zone Twill be located on a plane positioned 0.066 mm above the plane definedby the lip 114. At this location, the width of the toroidal zone is35.993 mm. The width of the cup 102 on a plane located 0.066 mm abovethe plane defined by the lip 114 is 36.0998 mm. Thus, the clearance is0.1068 mm.

At the plane defined by the lip 114, however, the width of the toroidalzone T decreases to 35.9927 mm while the width of the cup increases to36.1 mm resulting in a clearance of 0.1073 mm. In contrast, a preciselycircular ball with a radius of 18.035 mm would result in a clearance atthe plane defined by the lip 114 of 0.0300 mm.

Referring to FIG. 11, the femoral ball 106 is rotated within theacetabular cup 102 to the maximum amount possible before dislocationwould occur in an implanted device. The contact area between the ball106 and the cup 102 for purposes of this example is centered at location196. This configuration, which is not a normally occurringconfiguration, provides insight into the smallest expected clearance forthe embodiment of FIG. 3. That is, as the contact area is located morefully within the cup 102 with the ball 106 rotated as shown in FIG. 6,the origin 140 of the spherical cap portion moves off of the planedefined by the lip 114, thereby increasing the clearance at the lip 114.The width of the ball 106 in the plane defined by the lip 114 in theconfiguration of FIG. 6 is 36.04699 mm resulting in a clearance of0.0530 mm. In contrast, a precisely circular ball with a radius of18.035 mm in the configuration of FIG. 6 would result in a clearance atthe plane defined by the lip 114 of 0.0300 mm.

Thus, while the configuration of the prosthetic hip joint 100 providesthe desired conformity between the ball 106 and the cup 102 regardlessof the orientation of the ball 106 within the cup 102, the conformity isachieved while providing increased clearance on the plane defined by thelip 114.

An alternative embodiment of an acetabular cup 200 is shown in FIG. 12.The acetabular cup 200 includes an outer surface 202 and an innersurface 204. The inner surface 204 includes a cap portion 206 formed ona circle 208 with a center 210 and a toroidal portion 212. The toroidalportion 212 is shown in cross-section as formed on two circles 214 and216 having centers 218 and 220, respectively. The center 208 is locatedabove the axis 222 defined by the centers 218 and 220. In thisembodiment, the toroidal portion 212 is formed on the apple or outersurface of the torus defined by the rotation of the circles 214 and 216.Accordingly, even if each of the circles 208, 214 and 216 have the samediameter, the diameter of the cup 200 in the toroidal portion 212 willbe greater than the diameter in the cap portion 206.

As noted above, a cup and head are generally aligned in the implantedposition such that the apex of the cup is about thirty degrees off theaxis of the head in the coronal plane and about fifteen degrees off theaxis of the head in the sagittal plane. Accordingly, it may be desiredto modify the location of the cap portion of a cup. For example, FIG. 13shows an acetabular cup 230 includes an outer surface 232 and an innersurface 234. The inner surface 234 includes a cap portion 236 formed ona circle 238 with a center 240 and a toroidal portion 242. The toroidalportion 242 is shown in cross-section as formed on two circles 244 and246 having centers 248 and 250, respectively. The center 238 is locatedabove the axis 252 defined by the centers 248 and 250.

In this embodiment, the toroidal portion 242 is formed on the apple orouter surface of the torus defined by the rotation of the circles 244and 246. Accordingly, even if each of the circles 238, 244 and 246 havethe same diameter, the diameter of the cup 230 in the toroidal portion242 will be greater than the diameter in the cap portion 236.Additionally, the cap portion 236 is centered at a location 254 which isoffset from the apex 256 or deepest portion of the cup 230. Thus, thecap portion 236 is centered on the normal contact area between a balland the cup 230 when the ball and cup 230 are implanted. Accordingly,most of the contact between a ball and the cup 236 when implanted willoccur within the cap portion 236.

Although the present invention has been described with respect tocertain preferred embodiments, it will be appreciated by those of skillin the art that other implementations and adaptations are possible.Moreover, there are advantages to individual advancements describedherein that may be obtained without incorporating other aspectsdescribed above. Therefore, the spirit and scope of the appended claimsshould not be limited to the description of the preferred embodimentscontained herein.

1-16. (canceled)
 17. A prosthetic ball for use in a ball and cup jointsystem comprising: a spherical cap articulation portion defined by afirst radius of curvature; and a toroidal articulation portion definedby a second radius of curvature having a circular origin such that thetoroidal articulation portion is defined by a portion of the innersurface of a spindle torus.
 18. The prosthetic ball of claim 17, whereinthe origin of the radius of curvature of the spherical cap portion isspaced apart from a plane in which of the circular origin of thetoroidal articulation portion lays by less than 0.08 millimeters. 19.The prosthetic ball of claim 18, wherein: the origin of the radius ofcurvature of the spherical cap portion is spaced apart from the plane;and the circular origin of the toroidal articulation portion has adiameter that is less than the minimum distance between the origin ofthe radius of curvature of the spherical cap portion and the plane ofthe circular origin.
 20. The prosthetic ball of claim 19, wherein thediameter of the circular origin is less than 0.04 millimeters.
 21. Theprosthetic ball of claim 18, wherein the origin of the radius ofcurvature of the spherical cap portion is spaced apart from the plane bygreater than about 0.05 millimeters.
 22. The prosthetic ball of claim17, wherein the circular origin is located on a plane that isperpendicular to an axis extending through the origin of the firstradius of curvature and through the center of the spherical caparticulation portion.
 23. The prosthetic ball of claim 22, wherein theaxis intersects the plane at a location coincident with the center ofthe circular origin.
 24. The prosthetic ball of claim 22, wherein theaxis intersects the plane at a location offset from the center of thecircular origin.
 25. The prosthetic ball of claim 17, wherein thespherical cap articulation portion is defined by an opening angle ofless than about 125 degrees.
 26. The prosthetic ball of claim 17,wherein the spherical cap articulation portion is defined by an openingangle of about 95 degrees.
 27. A prosthetic cup for use in a ball andcup joint system comprising: a spherical cap articulation portiondefined by a first radius of curvature; and a toroidal articulationportion defined by a second radius of curvature having a circular originsuch that the toroidal articulation portion is defined by a portion ofthe outer surface of a spindle torus.
 28. The prosthetic cup of claim27, wherein the first radius of curvature has the same length as thesecond radius of curvature.
 29. The prosthetic cup of claim 27, whereinthe origin of the first radius of curvature is spaced apart from a firstplane in which the circular origin of the toroidal articulation portionlays.
 30. The prosthetic cup of claim 29, wherein the first plane issubstantially coplanar with a lower rim of the cup and the origin of thefirst radius of curvature is located above the first plane.
 31. Theprosthetic cup of claim 29, wherein the first plane is tilted withrespect to a second plane defined by a lower rim of the cup.
 32. Theprosthetic cup of claim 31, wherein the first plane is tilted by about30 degrees with respect to the second plane.
 33. The prosthetic cup ofclaim 32, wherein the origin of the first radius of curvature is withinan area bounded by the first plane, the second plane, the spherical caparticulation portion and the toroidal articulation portion.
 34. Theprosthetic cup of claim 27, wherein the circular origin is located on aplane that is perpendicular to an axis extending through the origin ofthe first radius of curvature and through the center of the sphericalcap articulation portion.